The present invention generally pertains to frequency dividers and is particularly directed to an improved frequency divider for use as an electronic tag in a presence detection system.
A presence detection system utilizing a frequency divider as an electronic tag is described in United Kingdom Patent Application No. 2,017,454. Such system includes a transmitter for transmitting a scanning signal at a first frequency in a surveillance zone; an electronic tag including an active frequency divider for detecting electromagnetic radiation at the first frequency and for transmitting a presence signal in response thereto at a second frequency that is a submultiple of the first frequency; and a receiver for detecting electromagnetic radiation at the second frequency to thereby detect the presence of the electronic tag in the surveillance zone. Such electronic tags are attached to articles of which detection is desired for enabling detection of the presence of such articles in the surveillance zone. Such presence detection systems are useful for detecting shoplifting, as well for other applications.
A few examples of such other applications include detecting the presence of a person or vehicle carrying an electronic tag in a surveillance zone; detecting the presence of articles bearing electronic tags within a surveillance zone along an assembly line; detecting the presence of keys attached to electronic tags in a surveillance zone at the exit of an area from which such keys are not to be removed; and detecting the removal of sentitive and valuable materials, such as a computer tape containing a data base or computer program, from a secure area by detecting the presence of such materials having electronic tags attached thereto in a surveillance zone at the exit of the secured area.
The electronic tag is encased in a small card-shaped container that can be attached to an article in such a manner that it cannot be removed from the article without a special tool. When used in a shoplifting detection system, a sales clerk uses a special tool to remove the electronic tag from the merchandise that is paid for; and the surveillance zone is located near the doorway for enabling detection of articles from which the electronic tags have not been removed.
The electronic tag described in the aforementioned patent application includes a complex frequency divider that must be powered by an expensive long-life miniature battery.
A frequency divider that may be operated without a battery or any other external power supply that is suited for use as an electronic tag in a presence detection system is described in U.S. Pat. No. 4,481,428. Such frequency divider includes a first circuit that is resonant at a first frequency for receiving electromagnetic radiation at the first frequency; a second circuit that is resonant at a second frequency that is a sub-harmonic of the first frequency for transmitting electromagnetic radiation at the second frequency; and a semiconductor switching device having gain coupling the first and second circuits for causing the second circuit to transmit electromagnetic radiation at the second frequency solely in response to unrectified energy at the first frequency provided in the first circuit upon receipt of electromagnetic radiation at the first frequency.